read two files simultaneously

[Junsuk Shin]

Both of them.

Reading two 100M files in interleaved way with 16K buffer, 62MB/s

Reading two 700M files in interleaved way with 16K buffer, 9MB/s

Reading two 100M files in interleaved way with 1M buffer, 55MB/s
  get worse with large buffer somehow

Reading two 700M files in interleaved way with 1M buffer, 34MB/s
  get better with large buffer, but still difference, 55 vs 34

I cannot find the reason for this. gstat(8) also shows low rates when
reading large files in interleaved way but not for small files.

On Sun, Feb 22, 2009 at 5:20 PM, Wojciech Puchar <
wojtek at wojtek.tensor.gdynia.pl> wrote:

> That's true. Using bigger buffer will help, but it doesn't tell why reading
>> large size file is slower than reading small size file.
>>
>>  really slower? or just bigger difference with large files?
>
>
>
>
>> On Sat, Feb 21, 2009 at 5:56 PM, Wojciech Puchar <
>> wojtek at wojtek.tensor.gdynia.pl> wrote:
>>
>>  I'm just guessing inode structure, the physical file location on HDD
>>>
>>>> might be related to this. But, if I read only one file, the size
>>>> doesn't matter. Reading file (10M, 100M, 700M) gives constantly about
>>>> 70MB/s, and the weird thing happens when I read 2 files of big size.
>>>>
>>>>
>>> if you use O_DIRECT it's read from disk exactly as you specified, without
>>> readahead, so you do a lot of seeks.
>>>
>>> simply use bigger buffer like 1MB
>>>
>>>
>>
>>
>> --
>> Junsuk
>>
>>